1. Technical Field
This invention relates generally to transient recorders for time array detection in time of flight mass spectrometry, and more particularly to an integrating transient recorder incorporating methods of operation and apparatus for determining ion intensities only at expected arrival times of ion peaks within one or more transients.
2. Discussion
Since the earlier part of the twentieth century, mass spectrometry has been a vital tool for the analyst and the scientist. This technique utilizes the understanding that neutral molecules can be ionized and, when in a vacuum, the resulting ions can be manipulated by electric and magnetic fields and detected with great sensitivity. The response of an ion to the magnetic and electric fields is dependent on the mass-to-charge ratio of the ion so that the ions of a specific mass-to-charge ratio can be detected and the number of ions at each of many mass-to-charge ratios can be determined.
Mass spectrometers are classified on the basis of the way in which the ions of differing mass-to-charge ratios are distinguished from each other. Magnetic sector mass spectrometers separate ions of equal energy on the basis of their momentum as they are deflected or dispersed in a magnetic field. Quadrupole mass filters isolate ions based on their rate of acceleration in response to a high frequency RF field in the presence of a DC field. Ion cyclotron and ion trap mass spectrometers separate ions based on the frequency or dimensions of their resonant oscillations in AC fields. Potentially the simplest of all mass discriminators, time of flight mass spectrometers, separate ions based on the velocity of ions of equal energy as they travel from an ion source over a fixed dimension to a detector.
In the time of flight mass spectrometer the neutral molecules are ionized in high vacuum in an ion source. Subsequent to ionization, a packet or bundle of ions (i.e., an ion source extraction) is synchronously extracted with a very short voltage pulse. The ions within the ion source extraction are accelerated to a constant energy and they then traverse a field-free region. During this time the ions separate from one another on the basis of their velocity. The difference between the instant of detection for any ions in a source extraction and the instant of their extraction from the source, is exactly timed. From this time of flight information the mass-to-charge ratio of a particular ion can be readily determined if the energy of acceleration and the distance travelled of the ion are known. For a linear field-free time of flight mass spectrometer, the simple relationship KE=1/2mv.sup.2 is used to derive equations that will calibrate the mass-to-charge ratio of the ions that are detected. Even in the presence of retarding or reflecting electric or magnetic fields, the times of arrival for all ions can be readily calculated based on knowledge of the mass-to-charge values of only two ions and their exact arrival times at the detector.
Although relatively simple and straightforward in design and concept, the time of flight mass spectrometer has been limited in applications due to the failure to take advantage of the very high rate at which information is generated at the detector. Because ions having different mass-to-charge ratios may be present in each ion source extraction, they will strike the detector at different times depending upon their velocities. The detector output signal is then made up of a sequence of ion arrival responses where the square of the arrival time is related to the mass-to-charge ratios of the detected ions. In order to reduce the effects of the energy variations of the ions and to increase the sensitivity of detection, relatively high accelerating potentials are commonly used (in the range of from 1,000 to 3,000 volts). The speed of the resulting ions, when accelerated by these potential differences, is quite great and, hence, the time between the arrival of ions of sequential mass-to-charge ratios is very short, generally less than one microsecond. Within a few hundred microseconds after initiating the ion source extraction, even the heaviest ions of interest (i.e., ions having the lowest velocities) will have arrived at the detector. Thus, the detector signal comprises a very brief "transient" containing a series of pulses where the individual amplitudes and pulse times correspond to the number and mass-to-charge ratios of the ions within the ion source extraction. The first time of flight instruments utilized exclusively oscilloscopes with variable persistence in order observe the transient signal produced by repetitive ion source extractions. Since this was essentially an empirical method, it required a reasonably constant sample pressure in the ion source during measurement and, even with photographs of the resulting oscilloscope traces, calibration and quantitation of the ions was exceedingly difficult.
An alternate recording method of readout was developed that utilized the concept called time slice detection (TSD). In this concept, a type of boxcar integrator is utilized. A time delay is placed between the time of the extraction pulse which generates the ion source extraction and the gating (i.e., initiating operation) of the detector circuitry. The detector circuitry is typically gated (i.e., "turned on") for a very brief period (2-15 nanoseconds) which represents approximately a portion of the variation in the arrival times for ions of a single mass-to-charge ratio at the detector. Accordingly, a "snap-shot" of the detector activity over a short, specific time interval, after the extraction pulse, is produced. Slowly varying the time delay in the initiating operation of the boxcar integrator over many successive extractions allows a "scan" across all potential ion arrival times. This progressively increasing time delay throughout the region of all of the arrival times of the ions requires from 2 to 10 seconds to produce the desired mass-to-charge versus the relative ion abundance across the mass-to-charge range of 2 to 800. Typically, the detected information is fed to an analog recorder where a permanent record of ion abundance (i.e., ion quantity) versus time (i.e., mass-to-charge) is obtained. Since the inception of time of flight mass spectrometry, measurements of the oscilloscope trace and/or time slice detector devices have dominated the read-out mechanisms.
A variation of time slice detection allows the ion peak measuring system to be activated by the event itself (i.e., an ion or ions striking the detector). This form of detection is generally known in the art as time-to-digital conversion. In this method of data collection, a counter associated with each arrival time window is incremented when an ion arrives within that window with the assumption that no more than one ion is involved for each window. This approach is employed in situations where very little amounts of sample are used and the measurements are made over long periods of time employing ionization methods designed to produce only a single ion most of the time. Multiple time storage actions can be accomplished during a single transient enabling several single ion events to be recorded for each transient.
Several approaches have been employed to improve the efficiency of the data collection processes for time of flight mass spectrometers. These include the use of more than one box car integrator, with each being triggered to the extraction pulse and each integrating the ion current over a separate time "slice". In this manner, up to eight or more individually measured points may be made subsequent to each ion source extraction. These points may be fixed in their delay time corresponding to ions having specific, predetermined mass-to-charge ratios. In this manner, a technique called selected ion monitoring (SIM) is realized whereby the collection process for a small number of ions of varying mass-to-charge ratios is made quite efficient. This mode, however, only works in situations where the sample constituency is either known or anticipated so that full spectral information may be sacrificed.
Time slice detection has two serious drawbacks: it is relatively slow in the generation of the scans and only a fraction of the data or information striking the detector is saved and utilized. Thousands of source extraction pulses may be required to acquire the information that is inherent in each detector output transient. Two major advantages of time of flight mass spectrometry, its rapid generation of spectral information and its high efficiency of ion utilization, are thus obviated by time slice detection. As a consequence, various devices have been developed for Time Array Detection (TAD) in which all of the information in an individual transient may be captured and stored. These devices are called transient recorders or digital transient recorders.
With transient recorders or digital transient recorders, a bank of high speed registers is filled sequentially in time with the information from the detector during the course of a single transient. The time access is dependent upon the digitizing rate of a dedicated analog-to-digital converter (ADC) and is usually in the 100 MHZ to 1 GHz range. The information from multiple transients may be continuously summed in a high speed summing memory register bank in a time locked mode for a preset number of transients, at the end of which time the register bank will contain information sufficient for the production of a single mass spectrum. These approaches have been used in many successful applications where the sample introduction has been static. Their major drawback is that once the memory bank has been filled, data collection must be suspended while the data are transferred to other memory or to a computing device. Indeed, in several devices, data collection is continuously interrupted by the summing within the storage memory itself. These processes limit the rate at which new transients can be accepted. With all known devices, this rate limitation results in the use of only a small fraction of the potentially available sample data. These gaps in the collection process make this approach totally unusable for applications such as chromatography where the continuity of the time axis must be maintained. However, time array detection has found widespread utility in situations where ions can be created in time dependent or time controlled modes, such as by laser ionization, with a low repetition rate pulsed laser.
For chromatographic and other time dependent applications, a far more efficient approach involves the use of a device called an integrating transient recorder (ITR). This device is capable of digitizing data at a rate sufficient to capture all of the information (i.e., the complete ion source extraction) from each and every extraction of a high repetition rate ion source. Subsequent transients are summed in a locked time registry in one of two memory banks until a summation or integration period is reached. This summation process yields several benefits. It is a linear summation (i.e., unweighted) and hence the sum file itself can be used as a single file of ion intensity versus time which is transformed to ion intensity versus mass-to-charge ratio, and is stored as a mass axis scan file, commonly referred to simply as a spectrum. These data accurately represent the ion population existing in the source over the integration time. Since the moment of extraction is the same for all ions having various mass-to-charge ratios, there is no skewing of the relative ion intensities as a function of the mass-to-charge ratio which in other types of mass spectrometers is caused by changes in sample concentration in the ion source during the time required to scan through the desired range of mass-to-charge ratios. Additionally, sequential summation increases both the signal-to-noise ratio and the ultimate sensitivity of the measurements. Finally, the summation process itself acts as a time shift mechanism allowing the information within the transient to be collected at a very high frequency and the resulting summed transient spectrum files to be transferred, processed and stored utilizing the electronic circuitry and bus structure of a typical high speed computer system. The integrating transient recorder described above is the subject of U.S. Pat. No. 4,490,806, issued Dec. 25, 1984, and was the first device of its kind to enable continuous time array detection in time of flight mass spectrometry. The disclosure of U.S. Pat. No. 4,490,806 is hereby incorporated by reference just as if same were fully set forth herein.
The presently preferred implementation of the integrating transient recorder described above makes use of a 200 megasamples per second, 8-bit flash analog-to-digital converter. The synchronized A/D converter output data is stored in two banks of high-speed emitter-coupled logic memory (ECL). Successive transients are summed in a locked registry in one bank while the other bank is simultaneously being read out into the data bus for subsequent processing and storage. After a desired operator-selectable number of transients have been summed in one bank, the spectrum file information in it is read out while the other bank, which has been cleared, is now used to collect the incoming data. Thus, data collection is continuous over an indefinitely long time. This technique allows all of the information in every transient to be used in the creation of subsequent spectra. Additionally, since only 10 transients need to be summed in the typical time of flight mass spectrometer (10,000 extractions per second) in order to reach levels that can be processed by other than high speed ECL logic, the integrating transient recorder described above is capable of creating and processing up to 1,000 spectrum files per second. In typical operation, approximately only 20-25 spectra per second are adequate to follow the temporal variations in the analyte composition of the transients.
While the above described integrating transient recorder has proved to be a significant success, the recorder itself is physically large and its ECL logic consumes a fair amount of power which necessitates a built-in air conditioner. It is very complex, somewhat expensive to build, and quite sophisticated in its operation.
Accordingly, it is a principal object of the present invention to provide an integrating transient recorder apparatus and method for time array detection in time of flight mass spectrometry, for continuously and without interruption acquiring, collecting and processing the information present in an ion extraction source, where all of the ion source extraction is captured and utilized in the generation of spectra with considerably less complicated and less expensive physical components than heretofore accomplished. More specifically, it is a principal object to accomplish detection of each ion peak within a transient by generating information only at the precise times at which ion peaks within a transient are expected to be arriving at a detector, to thereby greatly reduce the amount of information utilized from the detector while still detecting every ion peak present in the transient.
It is another object of the present invention to provide an integrating transient recorder apparatus and method for time array detection in time of flight mass spectrometry in which the apparatus incorporates means for forming a plurality of delta-mass tables which each include a plurality of predetermined time delays corresponding to the varying mass-to-charge ratios of ions within the ion source extraction, which predetermined time delays are controllably applied to initiate operation of integrating and/or peak detection circuitry at the expected arrival times of ions within the ion source extraction, and further only for a predetermined time duration.
It is yet another object of the present invention to provide an integrating transient recorder apparatus and method which compensates for mass defects in the masses of ions within said ion source extraction such that operation of an integrator or peak detection circuit of said apparatus is initiated in accordance with a modified time delay to thereby compensate for the variance in the anticipated arrival time of the ions introduced by the mass defect.
It is yet another object of the present invention to provide an integrating transient recorder apparatus which includes a first integrator or peak detector circuit responsive to ion peaks within a transient where the ions have only odd numbered mass-to-charge ratios, and a second integrator or peak capture circuit which is responsive only to ions having an even mass-to-charge ratio, and where each of the first and second circuits includes independent analog-to-digital converters, independent buffers, and independent digital signal processors.
It is yet another object of the present invention to provide an integrating transient recorder apparatus having a plurality of integrators responsive to a transient generated by an ion source extraction pulse, where operation of each of the integrators is turned on only at predetermined times of arrival of a limited number of ion peaks within the transient, and further where the operation of the integrators is initiated sequentially, one at a time, by a multiplexer control circuit.
It is yet another object of the present invention to provide an integrating transient recorder which is not only capable of determining ion intensity, but also determining the time of arrival of ions at a detector after applying a source extraction pulse.
It is still another object of the present invention to provide a new analog-to-digital converter for use with the integrating transient recorder apparatus thereof which expands the range of measurement capability of an otherwise conventional 8 bit analog-to-digital converter, automatically, depending on the magnitudes of the ion peaks of each incoming transient.